The invention relates to thermoplastic film structures that are sealable and peelable, and to methods of manufacturing the same.
A peelable film structure is described in publication WO 96/04178. The film structure comprises (a) a core layer comprising an olefin polymer and (b) a heat-sealable layer comprising a blend of low density polyethylene (LDPE) and a material incompatible with the LDPE, such as an olefin polymer or a co- or terpolymer of ethylene, propylene or butene. The film structure can be heat-sealed to a plastic container to form the lid of the container, or to itself to form a package. It is disclosed that when used with a plastic container, the film structure can be readily peeled from the container in order to open it.
U.S. Pat. No. 5,358,792 discloses a heat-sealable composition comprising (a) from about 30 to about 70 wt % of a low-melting polymer comprising a very low density ethylene-based copolymer defined by a density in the range of about 0.88 g/cm3 to about 0.915 g/cm3, a melt index in the range of about 1.5 dg/min to about 7.5 dg/min, a molecular weight distribution (Mw/Mn) no greater than about 3.5 and (b) from about 70 to about 30 wt % of a propylene-based polymer.
U.S. Pat. No. 5,443,915 discloses an oriented, polyolefin film that has a white-opaque cold seal-receptive skin layer on one side of a core layer and a vacuum deposited metal layer on the other side of the core layer. The cold seal-receptive polyolefin layer contains: (a) a slip agent in an amount sufficient to provide not more than about an 0.4 coefficient of friction to the surface of such layer; and (b) from about 10 to 40 wt % of titanium dioxide. The core layer is substantially free of opacifying filler and/or opacifying voids. The other side of the film is metallized to an optical density of at least 1.5, whereby the white pigment in the sealing layer in cooperation with the metallization gives a strong white-opaque appearance to the cold seal receptive layer.
U.S. Pat. No. 5,482,780 discloses an oriented polymeric xcex1-olefin film having: an isotactic propylene homopolymer core; a cold seal release skin layer adherent to one side of the core; and a surface treated cold seal receptive layer or such treated layer with a cold seal cohesive composition over the surface treatment of said layer on the other side of the core. The cold seal release skin layer comprises a slip agent and a blend of two polymers, namely, an ethylene-propylene random copolymer containing about 2% to 8% of ethylene in such copolymer and an ethylene-butylene copolymer containing about 0.5% to 6% of ethylene in such copolymer. The cold seal receptive layer is of an ethylene-propylene random copolymer containing about 2 to 8% of ethylene.
U.S. Pat. No. 5,500,265 discloses a peelable film comprising (a) a core layer comprising an olefin polymer, (b) a skin layer on at least one surface of the core layer, the skin layer comprising: a blend of a butylene polymer with another olefin polymer or a polymer of butylene and at least one other olefin, and (c) a coating layer on the skin layer.
U.S. Pat. No. 5,716,698 relates to a peelable, oriented, opaque, multilayer polyolefin film comprising at least one opaque layer and a peelable top layer. All layers of the film are essentially vacuole-free. The opaque layer contains pigments having a mean particle diameter of from 0.01 to 1 xcexcm.
U.S. Pat. No. 5,997,968 discloses a multilayer polyolefin film comprising at least three coextruded layers comprising an opaque base layer, an intermediate layer, and an outer peelable surface layer composed of two incompatible polyolefins, wherein the intermediate layer contains at least 80% by weight of a polyolefin having a lower melting point or lower glass transition temperature than the polyolefin forming the base layer.
U.S. Pat. No. 6,231,975 discloses a sealable film that comprises: (a) an inner layer comprising an olefin polymer; (b) a sealing layer, and (c) a separable layer positioned between the inner layer and the sealing layer, the separable layer comprising (1) ethylene-propylene block copolymer or (2) a blend of polyethylene and another olefin which is incompatible with the polyethylene, specifically either (i) polypropylene homopolymer or (ii) ethylene-propylene block copolymer.
U.S. Pat. No. 6,248,442 relates to multilayer films that are heat-sealable over a broad temperature range. The ""442 patent also relates to multilayer films that provide easy-opening and hermetic seals to packages. The films of the ""442 patent are made up of a core layer comprising linear low density polyethylene (LLDPE) and at least one skin layer having a melting point of at least 10xc2x0 C. below the core layer melting point.
There is still room for improvement, however, in the field of sealable and peelable film structures. In particular, a need still exists for a film structure that maximizes the advantageous combination of (i) a sealable outer layer that seals well to itself or other surfaces over a broad temperature range and (ii) a particular multilayer design that permits (a) separation of the sealed outer layer from at least one other layer of the film and (b) the seal to be readily opened without creating a xe2x80x9cz-directionxe2x80x9d tear.
A z-direction tear disrupts the integrity of a multilayer film when the film is pulled apart at the seal. A film with a z-direction tear has not simply separated at the seal line. Instead, the separation, or tear, has extended to other layers of the film. It is difficult to properly reclose a package that has a z-direction tear, thereby hampering the ability of the package to maintain the freshness of its contents. When z-direction tears can be eliminated, packages are easily refolded and sealed by simple mechanical means, such as a clip.
It is an object of the invention to provide a thermoplastic film structure with a sealable outer layer that seals well over a broad temperature range.
It is another object of the invention to provide a thermoplastic film structure with a sealable outer layer that, when sealed, can be readily opened without creating a z-direction tear.
It is a further object of the invention to provide a thermoplastic film structure with multiple layers, including a core layer comprising a peelable, polyolefinic blend and a sealable outer layer that can be sealed to itself or other surfaces, wherein the particular multilayer thermoplastic film structure permits separation of the sealable outer layer from at least one other layer of the film.
It is yet another object of the invention to provide both clear and opaque thermoplastic film structures.
There is provided a sealable and peelable film, including at least (a) a core layer comprising a peelable blend comprising (i) from 20-80 wt % of a polyethylene and (ii) from 80-20 wt % of a polyolefin incompatible with polyethylene (i) and (b) a pair of outer layers, at least one of which is a sealable outer layer, each independently comprising a thermoplastic polymer.
There is also provided a sealable and peelable film, including at least (a) a core layer comprising a peelable blend comprising (i) from 10-35 wt % of an ethylene-xcex1-olefin elastomer and (ii) from 90-65 wt % of a propylene homopolymer and (b) a pair of outer layers, at least one of which is a sealable outer layer, each independently comprising a thermoplastic polymer.
There is also provided an opaque, sealable and peelable film, including at least (a) a cavitated first core layer comprising a thermoplastic polymer and a cavitating agent; (b) a second core layer comprising a peelable blend comprising (i) from 20-80 wt % of a polyethylene and (ii) from 80-20 wt % of a polyolefin incompatible with polyethylene (i); and (c) a pair of outer layers, at least one of which is a sealable outer layer, each independently comprising a thermoplastic polymer.
There is also provided an opaque, sealable and peelable film, including at least (a) a cavitated first core layer comprising a thermoplastic polymer and a cavitating agent; (b) a second core layer comprising a peelable blend comprising (i) from 10-35 wt % of an ethylene-xcex1-olefin elastomer and (ii) from 90-65 wt % of a propylene homopolymer; and (c) a pair of outer layers, at least one of which is a sealable outer layer, each independently comprising a thermoplastic polymer.
Sealable and peelable film structures according to the present invention may also include an intermediate layer comprising a propylene homopolymer, wherein the intermediate layer is positioned between the core layer comprising the peelable, polyolefinic blend and the sealable outer layer.
Once sealed, a sealable film without the present multilayer design is usually torn through, i.e., torn in the z-direction, to open the seal, especially when the area where the film is sealed is stronger than the film as a whole. With the sealable and peelable film structures of this invention, a seal can be opened without tearing the film in the z-direction because the particular multilayer structure of the present films, including a core layer comprising a peelable, polyolefinic blend, permits the seal to be separated without tearing the film in the z-direction.
Thus, a particular advantage of the present sealable and peelable films is that they do not compromise the desired properties of a seal material, such as hot tack and seal strength, because the present sealable outer layer(s) may use ordinary seal materials to seal the film, and yet the films are still peelable.
For example, a sealable and peelable film structure according to the present invention may advantageously possess a peak crimp seal strength between 300 g/in and 800 g/in, e.g., from 450 g/in to 750 g/in, and a ratio of plateau crimp seal strength to peak crimp seal strength of greater than 60% and less than 100%, e.g., greater than 70% and less than 95%. These ranges identify a seal that is both strong and readily peelable.